The invention is based on a fuel injection pump. In a fuel injection pump of this kind known from German Auslegeschrift No. 1 576 617, a deflecting piston is provided which defines a fuel withdrawal chamber connected via a throttle with the pump work chamber. The deflecting piston is restored by a compression spring supported on a spring plate embodied in the manner of a piston. Coaxially with the compression spring, a stop tang protrudes inward from the spring plate, blocking the deflecting piston after a certain amount of compression has taken place on the part of the restoring spring. The piston-like spring plate is pressed by the compression spring against a stop within a cylinder bore and from the opposite side is acted upon by a pressure medium at a rpm-dependent pressure. If this pressure exceeds the tension of the restoring spring, then the piston-like spring plate is displaced far enough that the deflecting piston is blocked. This apparatus for displacing the piston-like spring plate serves to switch off the movement of the deflecting piston in an rpm-dependent manner. With the aid of the deflecting piston and of the withdrawal chamber connected via the throttle with the pump work chamber, the course of the injection rate per crankshaft angle during one injection which is attained by the known apparatus is in steps or stages. After the blocking of the deflecting piston, the otherwise usual course of injection is supposed to be adhered to. In a different embodiment, beyond a predetermined stroke distance an additional valve opens up and additional connection at the deflecting piston for communication with the pump work chamber. The result in the known apparatus is that the injection pressure is so severely reduced that the injection valve closes in the meantime, preventing further fuel from being injected. With an apparatus of this kind, a pre-injection which is separate from the main injection is attained. Beyond a predetermined rpm, the deflecting piston is again blocked, so that fuel then continues to be injected in the usual manner.
Such devices serve to make the course of combustion smoother, and particularly in self-igniting engines with fuel injection directly into the combustion chamber they serve to prevent too much fuel from collecting in the combustion chamber from the time that the first injection begins until the ignition delay period has elapsed; such excess fuel then combusts abruptly. This abrupt combustion causes a steep increase in pressure and is accordingly associated with considerable noise during combustion.
Aside from the above known apparatus, other injection nozzles with which a pre-injection can be realized are also known. Such nozzles are associated with considerable expense, however, as compared with a disposition of the type initially discussed above.
The apparatus of the type initially discussed above is also relatively expensive in terms of its embodiment, in which a pre-injection which is offset from the main injection in terms of time is supposed to be attained, and in its operation it is highly dependent on the rpm. With increasing rpm, the throttle connection between the withdrawal chamber and the pump work chamber has an increasingly pronounced effect, as do all the other controlled connections. The pre-injection thereby attainable thus varies widely in response to dynamic operating conditions. Although this influence is rendered ineffective in the known apparatus by blocking the deflecting piston beyond a certain rpm, still the known apparatus accordingly has the disadvantage that it is effective over only a small portion of the entire operating range.